Loss of interferon-γ inducibility of the MHC class II antigen processing pathway in head and neck cancer: evidence for post-transcriptional as well as epigenetic regulation

Summary Background Abnormalities of the major histocompatibility complex (MHC) antigens by tumour cells impair the cellular immune response and promote tumour evasion from immune surveillance. So far, studies analysing the MHC class II expression levels in head and neck cancer have been limited. Objectives Therefore, we investigated the constitutive and interferon (IFN)‐γ‐regulated expression profiles of MHC class II antigen processing machinery (APM) in various head and neck cancer cell lines and also analysed the MHC class II expression in head and neck cancer lesions. Methods Using immunohistochemistry, flow cytometry, and reverse transcriptase‐polymerase chain reaction analyses we investigated the expression pattern of various components of the MHC class II APM in biopsies and cell lines from head and neck cancers. Furthermore, we analysed the class II transactivator (CIITA) and HLA‐DR promoter activity in head and neck cancer cells by transient transfection of specific luciferase promoter constructs and finally studied the methylation pattern of the CIITA promoter using methylation sensitive restriction enzymes. Results Head and neck cancer cell lines analysed in vitro lacked constitutive MHC class II surface expression. Despite the IFN‐γ‐mediated induction at the mRNA level, six out of 10 cell lines did not show any relevant MHC class II surface expression. This phenomenon might be attributed to a post‐transcriptional dysregulation of specific MHC class II APM components. One cell line displayed a loss of IFN‐γ‐induced CIITA‐expression that corresponded to impaired MHC class II surface expression, which could be linked to hypermethylation of the IFN‐γ‐responsive CIITA‐promoter IV. In vivo, immunohistochemistry analyses of 35 patients revealed that about 86% of head and neck cancer tissues exhibit a negative or only marginally positive staining, whereas 14% displayed a heterogeneous or highly positive MHC class II surface expression. There was no statistical correlation between tumour differentiation and the MHC class II expression in head and neck cancer lesions. Conclusions Taken together these results suggest a high frequency of MHC class II abnormalities in head and neck cancer in vitro and in vivo, which could occur at different steps of the antigen processing pathway. This information may have a significant impact on practical and clinical aspects of tumour immunotherapeutic strategies.